Teach him to use Anki. If I would have had started to learn via anki when I was 8-year-old I would have so much knowledge in my head.
Much of that knowledge would be useless, though. Aren't you worried about your brainspace?
I love Anki, but it would be so weird to have Anki cards that my 8 year old self made. Just stick to simple facts if you use it, or it'll be a mess.
ETA: Thinking about public schooling and Anki is really prone to sunk cost fallacy, I think. Just because you've wasted time learning things now forgotten, doesn't mean the effort you'd have spent keeping them in your memory would have been worthwhile.
Although my 8-year-old son likes his teacher, he is frequently bored at school. He attends a high quality suburban public school in the United States. He has a lot of traits in common with LessWrong readers, and we would like advice for what he can do to counter his boredom. Many of you must have found grade school more or less tedious. What were your coping strategies?